Induction heating of recording styli



Feb. 10, 1953 s ow 2,628,104

INDUCTION HEATING OF RECORDING STYLI Filed March 31, 1950 INVENTOR Jazz/M1100 X. JZzrd/azzz ATTORNEY Patented Feb. 10, 1953 INDUCTION HEATING OF RECORDING STYLI Lawrence R. Shardlow, North Arlington, N. .L, assignor to Radio Corporation of America, a

corporation of Delaware Application March 31, 1950, Serial No. 153,232

4 Claims.

This invention relates to sound recording apparatus employing a heated recording stylus, and more particularly to high frequency induction heating of such stylus.

It has been found that when a recording is made with a heated stylus, the background noise is much less than when the recording is made with a stylus at ambient room temperature. Although different methods have been proposed heretofore for heating the recording stylus, such, for example, as radiant heating or resistance heating, they possess certain disadvantages in cases where high fidelity recordings are desired.

In the case of radiant heating, where a coil is disposed in closely spaced relation about the stylus, the coil is heated to a bright red condition. Lacquer chips from the recording disc frequently come in contact with the coil and are ignited. On the other hand, in the case of resistance heating, where the stylus is part of an electric circuit, it is required that the stylus be an electrically conductive member. This method obviously cannot be used where it is desired to use a sapphire recording stylus or some other electrically non-conductive stylus.

To overcome the above-mentioned difficulties, it has been proposed, in the copending application of Schoemehl et al., Ser. No. 153,248, filed on March 31, 1950, to heat the stylus inductively by means of a heating coil connected to a source of high frequency current. In such a method, the relation of the heating coil and the article to be heated is important from the standpoint of efiiciency. Since induction heating is accomplished by disposing the workpiece in an alternating magnetic field, it is desirable to utilize as much of the useful magnetic fiux in that magnetic field as possible.

It is the primary object of the present invention to provide an efficient apparatus for inductively heating recording styli.

It is another object of the present invention to provide induction heating apparatus which concentrates the high frequency field on a small area.

It is also an object of the present invention to provide induction heating apparatus for sound recording instruments which permits mounting the source of power independently of the recording head.

A further object of the present invention is to provide a simple and highly efficient induction heating apparatus which is particularly adapted for heating recording styli.

In accordance with the present invention a magnetically permeable, electrically non-conductive core is disposed within a coil through which high frequency current may be passed. The coil is inductively coupled with an electrically conductive means comprising at least a portion of the length of a recording stylus. One end of the core is extended outwardly from the coil and disposed in close proximity to the conductive portion of the recording stylus thereby to concentrate the magnetic field produced by the coil onto the conductive portion. With this arrangement, it is possible to mount the heating source independently of the recording apparatus so that it wil1 not interfere with the actual recording.

The novel features characteristic of the present invention, as well as additional objects and advantages thereof, will be better understood from the following detailed description when read in connection with the accompanying drawing in which:

Figure 1 is a view of one form of heating apparatus in accordance with the present invention as applied to one type of sapphire recording stylus,

Figure 2 is a view of another type of sapphire recording stylus which can be used with the heating apparatus shown in Figure 1,

Figure 3 shows a metal stylus which also can be used with the heating apparatus shown in Figure l, and

Figure 4 is a sectional view of the coil and core taken on the line 4-4 of Figure 1.

Referring more particularly to the drawing, wherein similar reference characters designate corresponding parts throughout, ther i shown, in Figure 1, apparatus I for inductively heating a recording stylus 3. The apparatus I comprises a coil or winding 5 having the terminal leads I thereof connected to a suitable source 9 of alternating high frequency current, such as an oscillation generator operating at radio frequency and suitable for induction heating.

A core II of magnetically permeable material having high electrical resistance and also having a Curie temperature above the operating temperature of the coil 5 is disposed axially through the coil. One end I3 of the core I I extends outwardly from the coil 5. This end is tapered in order to concentrate a substantial part of the magnetic flux of the high frequency magnetic field produced by the coil onto the electrically conductive portion I4 of the stylus. While the end I3 may be tapered in any desirable manner, it is found preferable to taper it wedge-like. In other words, the core tapered end I3 is truncated, as shown particularly in Figure 1. Effectively, the tapered end I3 wil] concentrate a substantial amount of the magnetic flux of a field produced by the coil within a space defined by lines drawn as extensions of the surface of the tapered end (for example, space within the broken lines I6 shown in Figure 1). Thus, it will be recognized by persons skilled in the art that the area over which the magnetic field may be concentrated by the core tapered end I3 may be controlled by varying the degree of taper, as well as by varying the cross sectional shape of the tapered end.

tion of the resin bond, thus making them conductive. Ferrites, on the other hand, notably copper ferrite, are found excellent because they have a high Curie temperature (often 450 C. or higher) and remain magnetically susceptible up to that temperature. Copper ferrite may be made as described in U. S. Patent, 1,946,964.

The coil 5 and core I l are mounted in any convenient manner so as to direct the core tapered end 13 toward the target 14 of electrically conductive material which comprises a part of the stylus t. The mounting of the stylus 3 is such as to dispose the target within the concentrated portion of the magnetic field produced by the re tapered end !3. Accordingly, the coil and core I! may be mounted on the recording arm which supports the recording stylus independently of the recording head. They may even be mounted on a separate support so that they will not add to the mass of the recording arm.

Inasmuch as the stylus to be heated may comprise a nonconductive material, such as sapphire, it is necessary to provide a target of electrically conductive material thereon so that a shorted turn coil will be provided as a secondary winding for inductive coupling with the coil 5 as a primary winding. For this purpose, it has been found preferable to deposit or otherwise provide a thin metallic layer or coating it on the surface of the stylus, the layer i l surrounding at least a portion 0a of the length of the stylus and being disposed in close proximity to the groove cutting edge :5. A stylus of this type may comprise an electrically non-conductive body of sapphire material upon aportion of which a layer of silver or gold is first deposited, as by evaporation or other process. A second layer of metal of high magnetic permea bility, such as nickel or iron, is deposited over the first layer, as by electroplating. Such a stylus is more particularly described and claimed in the above-identified copending appiication of Schoemehl et al., and is also illustrated in Figure 1' of the accompanying drawing. Although the stylus 3 may be supported in any suitable manner from its opposite end i i, the support l8 should be made from material which will not be affected by heat.

Another arrangement for providing an electrically conductive target on the non-conductive stylus 3 is to closely fit a suitable band or sleeve [9 of iron or the like about a portion of the stylus adjacent to the groove cutting edge 55. Such an arrangement is shown more particularly in Figure 2 of the drawing.

On the other hand, if the stylus comprises an electrically conductive material, such as the metal stylus 2i illustrated in Figure 3 of the drawing, the stylus itself is an excellent medium for transfer of electrical energy by induction. In such a case, the tapered end it of the core ll need only be directed toward the body of the stylus S at a portion of the surface thereof which is disposed adjacent to the cutting edge IS.

The size or" the heating coil 5, the size and composition of the core H, and the source of high frequency energy may be of any values which are suitable for inductively heating a recording stylus. In order to give some idea of values which have been found satisfactory for inductively heating a recording stylus, one heating coil comprised a two layer winding of A; inch copper tubing having a core with a tapered end disposed axially therethrough. The inner and outer layers were connected to form a continuous tube through which a fluid was passed to efiect necessary cooling. The inner layer of the windin comprised 5 turns of the tubing wound in contact with and around the core. The outer layer comprised two turns of tubing wound around the inner layer adjacent the tapered end of the core. The core was made {a of an inch in diameter and of copper ferrite material. A current of about to amperes at 300 to 325 kilocycles per second was fed through the coil.

Thus, the present invention provides an apparatus for inductively heating a recording stylus which is simple and eifective. In addition, the apparatus possesses the advantage that it can be mounted independently of the recording head, thereby preventing a damping efiect on the stylus which otherwise would interfere with accurate recording. The core also concentrates the magnetic flux where it is needed, thereby increasing the efliciency over conventional open core types'of heating coils.

It will be obvious to persons skilled in the art that various modifications and changes in the induction heating apparatus shown and described herein are possible within the spirit of the present invention. It is, therefore, desired that the particular form of the invention described herein shall be considered as illustrative and not as limiting.

What is claimed is:

1. Apparatus for recording sound vibrations upon a record with a heated stylus comprising, in combination, an electrically non-conducting stylus, an electrically conducting material fixed to said stylus intermediate the ends thereof, and a coil having a magnetically permeable core axially disposed therethrough, said coil and core being mounted with one end of said core disposed in closely spaced relation to said conducting material.

2. Apparatus for recording sound vibrations upon a record with a heated stylus comprising, in combination, an electrically non-conducting stylus, an electrically conducting material fixed to said stylus intermediate the ends thereof, a coil having a magnetically permeable core axially disposa therethrough, said coil and core being mounted with one end of said core disposed in closely spaced relation to said conducting matesaid conducting material being in the form of a band closely fitted on said stylus.

3. Sound recording apparatus according to claim 8 wherein said one end of said core is tapered and extends outwardly from said coil.

4. Sound recording apparatus according to claim 8 wherein said core is a copper ferrite.

LAV /TRENCE R. SHARDLOW.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,649,847 Nightingall Nov. 22, 1927 2,378,890 Abe et a1 June 26, 1945 

